QuestionCan a potato petrify? I've Googled the subject of "potato rock" and it seems that there are a lot of these rocks that resemble red potatoes around. One article "Petrified Potatoes Upset Housewives," The New York Times, 10/71912, described children finding "potato rocks" at an excavation for the New York subway system and placing them with a grocer's real potatoes. A friend from north-central Oregon found one and said an expert said it was a fossilized potato, but I don't think potatoes were around long enough for fossilization.

AnswerThe rocks you describe are not petrified potatoes. First potatoes were indigenous to South America. To my knowledge they have not been around in the US long enough to become petrified.

Second New York was the site of glaciation. Manhattan is actually a terminal moraine. So it is more probable that the potato rocks were rounded pieces of chert or a similar rock. Or a concretion. Chert, chalcedony and flint are a micro crystalline form of quartz. It is hard and dense and makes up a lot of the rounded rocks found in high energy streams. Rounded rocks are very common in glacial moraine and till material.

Glacial deposits are interesting. Think of a glacier as a conveyor belt. It grows on one end high in the mountains, and melts at the other end when it reaches a lower elevation where the ice can melt.Ice in a glacier flows just like a river and has tributaries that flow together to form the core or main part of the glacier. They move from the high area to the low area under the weight of the ice. If the glacier happens to reach the ocean, instead of melting it breaks off chunks we call this calving icebergs. If you look at an aerial photo of a glacier the black stripes on the glacier are rock and debris from along the sides of the mountains and valleys. If you count the number of stripes and divide by 2 you will know how many tributaries merged to form the main glacier.

Okay, well the rocks move along with the ice and when the ice melts, it drops the rock and debris in piles called moraines. The Wisconsin Dells area are hills formed by glaciers that stayed stationary and dropped lots of soil and rock. If a glacier advances and retreats the thick layer of material it lays down is called glacial till. It consists of soil, rock and sand. Other deposits called loess are distributed by wind. The whole upper mid west agricultural area of the US has loess deposits and this accounts for the great fertility of the region.

Once dropped by the glacier, the rocks are carried along in streams of melt water and become what we call outwash. If the glacier advances over these deposits it bulldozes them up into new moraines. this material can be worked and reworked several times.

So, the potatoe rocks were probably rocks moved by glaciers and worked by streams forming smooth potato shaped rocks. The red ones were undoubtedly chert or flint with a red color that looked like red potatoes.

Oregon underwent similar glaciation. Vast floods took place that drained water from a lake that formed along the margin of the ice lying in western Montana. It was called Lake Missoula. The water drained from there through Washington State to the Pacific ocean in a flood that boggles the imagination. It formed a waterfall that is dry today, but made Niagara Falls look like a trickle. It scoured soil and rock from a vast area exposing volcanic lava flows that are still visible from space. The water flow formed ripples that look like hills and moved rocks along the size of houses.

My point is that where ever you find glacial material, you will find potato like rocks. The fact that they are dug out of soil, much like a potato, gives rise the the idea that they are petrified potatoes.

There are other rounded stones that are called gastroliths. We know that dinosaurs are related to birds and we know that birds need gravel in their crops and gizzards to grind seed. We see this is other modern analogs, crocodiles, alligators, seals and sea lions. These animals use them more for ballast than for aiding mastication of food. This gave rise to the idea that some dinosaurs also needed rocks in their crops or stomachs to grind their food or use a ballast. To meet the gastopod classification there are criteria that must be met, like the rock cannot be like any other rock in the vicinity, it must be found in proximity to dinosaur bones etc. So that rules out potato rocks found in modern soils.